Current Balance Capacitor – Understanding Its Role

balancingcapacitorcurrent

Most series capacitors needs balancing resistors to keep the same voltage across them, but in my case, I'm using with three capacitors of 45uF in parallel that are working very close to its maximum RMS current rating. I'd like to know, is it necessary to put any resistor or inductor to balance the current of the capacitors? If so, could you please guide me on how to do that.

Best Answer

An exact answer would depend on many other factors, so it is possible that engineer A has an opinion which does not conclude with the opinion of engineer B.

I can't answer whether you need to add any resistors, but my answer(s) will hopefully lead you to the right path:

  • If possible, just buy a higher rated component.
  • The rated operating parameters are stated for the all possible variations during production. Being at or slightly above the limit will not cause malfunction in most of the cases. It will have a finite probability though. You have to decide how much risk you are willing to take.
  • If the caps are from the same rail, then they will have similar parameters with a very high probability, so unless there is a significant temperature gradient, no balancing is needed. Here was also an assumption, but a fair one.
  • If none of the aboves are not the case, you should check the capacitance value accuracy and the ESRs and ESL. The PCB traces will contribute to it as well. Also a question is whether you would like to design for worst case, or just for acceptable yield. The last case would require some tests.

Note: that any resistance in series with the cap reduces its quality factor, and increase its losses, and reduce the peak currents it is capable of supply. So try to avoid it if possible.

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